The present invention relates to brake actuators, and particularly to such brake actuators as incorporate a combined back-up emergency and parking brake in conjunction with the primary service brake.
In one such brake actuator, the service brake is air-applied, spring-released; and the emergency/parking brake is spring-applied, air-released. A lost-motion coupling between the service and emergency brake pistons allows either one or both pistons to be effective to apply braking force. By utilizing a common source of air for the service and emergency brakes, failure of the service brake due to loss of air, for any reason, such as insufficient compressor capacity, automatically results in the emergency brake being applied under spring force. Typically, force amplification levers are employed to obtain high braking force without increasing cylinder size. This, in turn, requires the use of a slack adjuster to compensate for excessive piston travel throughout the range of brake shoe wear.
A typical slack adjuster used with the aforementioned brake actuator employs a toothed ratchet wheel that is arranged to effect rotation of an axially fixed nut member having threaded engagement with a brake actuator rod that is fixed against rotation. A pawl is actuated to engage a tooth of the ratchet wheel when the piston stroke exceeds a nominal distance during a brake application. During a brake release, a spring connected to the pawl pulls it in a direction tangential to the circumference of the ratchet wheel teeth to effect rotation of the ratchet wheel and, consequently, of the nut member. Since the threaded actuator rod is fixed against rotation, this rotation of the nut member forces the actuator rod to advance axially and thereby take up excess brake shoe clearance. The slack adjuster is designed to operate in increments of one tooth at a time. Since the maximum distance that the pawl shifts is determined by the maximum piston stroke, the continuous slack adjuster action will assure that the pawl does not normally shift a distance greater than that necessary to engage a single tooth at a time.
The aforementioned brake actuator is also provided with a manual release mechanism for releasing the parking brake when no air is available. This manual release mechanism operates through the brake actuator rod assembly to shorten the brake linkage which, in turn, causes the service and emergency pistons to bottom out. Once this occurs, continued operation of the manual release mechanism forces the actuator rod assembly to retract the brake shoes. It will be appreciated, however, that in bottoming out during this manual release operation, the service and emergency pistons move a distance sufficient to cause the slack adjuster pawl to shift several teeth over the ratchet wheel. This results in the line of action of the pawl swinging from its normal tangential alignment relative to the ratchet wheel teeth to a generally radial alignment relative to the ratchet wheel, thereby becoming jammed to render the slack adjuster inoperable.
Moreover, the high compressive forces acting on the brake linkage when the service and emergency brakes are both applied, result in over-travel of the brake actuator pistons due to slight bending of the linkage components, thus giving a false indication of excessive brake shoe wear. The piston over-travel resulting from this apparent excessive brake shoe wear causes the slack adjuster to over-extend the brake linkage in attempting to compensate for slack that does not actually exist. Accordingly, insufficient brake shoe clearance and possible brake lock-up can occur.